Lifetime extending and performance improvements of optical fibers via loading

ABSTRACT

A method of making a microstructured optical fiber including loading the core and cladding materials of the fiber with hydrogen and deuterium at a loading temperature; annealing the fiber at a selected temperature T anneal ; pumping the fiber with radiation; and reducing the temperature of the fiber and storing the fiber at the reduced temperature before the step of pumping the fiber; and wherein the method allows the hydrogen and the deuterium to become bound to the core material and the cladding material.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.16/658,747, which was filed in the U.S. Patent & Trademark Office onOct. 21, 2019, and which will issue as U.S. Pat. No. 11,048,145, whichis a continuation of U.S. application Ser. No. 16/256,096, which wasfiled in the U.S. Patent & Trademark Office on Jan. 24, 2019, now U.S.Pat. No. 10,474,003, and which is a continuation of U.S. applicationSer. No. 15/963,694, which was filed in the U.S. Patent & TrademarkOffice on Apr. 26, 2018, now U.S. Pat. No. 10,281,797, which is acontinuation of U.S. application Ser. No. 15/683,281, which was filed inthe U.S. Patent & Trademark Office on Aug. 22, 2017, which is acontinuation of U.S. application Ser. No. 13/003,510, which was filed inthe U.S. Patent & Trademark Office on Jan. 10, 2011, now U.S. Pat. No.9,746,749, which is a national stage application of PCT/DK2009/050158,which was filed on Jul. 3, 2009, and which claims the priority of DanishPatent Application No. PA 2008 00985, which was filed on Jul. 11, 2008.The contents of U.S. application Ser. Nos. 16/658,747; 16/256,096,15/963,694; 15/683,281; 13/003,510; PCT/DK2009/050158; and Danish PatentApplication No. PA 2008 00985 are incorporated herein by reference.

TECHNICAL FIELD

The invention relates specifically to an optical fiber comprising a coreand a cladding comprising a core material and a cladding material,respectively.

BACKGROUND ART

Guiding of relatively high powers in an optical fiber may have relevancefor several commercial applications such as guiding of surgical and/ortherapeutic light, optical sensing, and materials processing. Among suchapplications is transport of optical energy and utilizing non-lineareffects in the fiber, which are commonly more pronounced with higheroptical power inside the fiber. The optical power may be continuous wave(CW), pulsed or a mixture thereof. High optical power inside a fiber maybe particularly pronounced with pulsed light where a high peak power maybe obtainable even while having a relatively modest average power.

One limitation of the average power/spectral density carried by anoptical fiber is the damage threshold of the fiber. The input facet orthe first few millimeters of fiber may be destroyed if the optical power(CW, peak power or pulse energy) is above the bulk glass or glass-airinterface damage threshold. It has been observed by the presentinventors that even when the optical power is below this threshold theoptical fiber may still be observed to degrade over time. Thisdegradation is often observed as increased absorption in the visibleover time. For commercial applications a long lifetime may be critical.

SUMMARY

An object of the present invention is to provide a fiber where suchdegradation is either eliminated or reduced.

The objects of the invention are achieved by the invention described inthe accompanying claims and as described in the following.

One object of the invention is achieved by an optical fiber comprising acore and a cladding comprising a core material and a cladding material,respectively, said optical fiber being obtained by a method comprisingloading said core material and optionally said cladding material withhydrogen and/or deuterium under loading conditions suitable to allowhydrogen and/or deuterium to bind chemically to said material(s). In oneembodiment said loading condition comprises at least one of a) a raisedtemperature T, b) a raised pressure P c) irradiation and/or d)subsequent irradiation. Such an optical fiber may have particularsuitable properties in regard to long lifetime in applications wheresaid fiber is arranged to guide relatively high power, such as pulsedlight with relatively high peak power.

In one embodiment of the invention degradation of the fiber in asupercontinuum light source for shorter wavelengths is reduced bystripping of higher order modes. Accordingly, in one embodiment theinvention relates to a supercontinuum light source comprising a fibercoupled to a pump source where said pump source and said fiber areadapted to provide an optical output spanning over at least one octavewith at least 10 μW/nm, and/or wherein said pump and said fiber areadapted to provide a maximum modulation instability gain Ω_(max) largerthan 20, wherein at least part of said fiber is arranged to strip higherorder modes and/or suppress coupling of light from the fundamental modeto higher order modes. In one embodiment this stripping of higher ordermodes is combined with loading of the fiber with deuterium and/orhydrogen.

Loading by deuterium is sometimes applied in the art in order toovercome absorptions in the so-called water-band, which increases whenthe fiber is subjected to hydrogen-rich environments, such as found forundersea communication cables. This issue is not similar to the presentproblem. In one embodiment the fiber is therefore applied in anenvironment where it is subjected to a medium with a content of H₂and/or H⁺-ions of less than 5 at %, such as less than 1 at %, such asless than 0.1 at %, such as less than 0.01 at %, such as less than 0.001at %.

Loading by deuterium is sometimes applied in the art in order toincrease the photosensitivity of the fiber in order to enable changingits refractive index, e.g. to induce a Bragg grating in the fiber byexposure through an external light source perpendicular to the fiber.This issue is not similar to the present problem. In one embodiment thefiber is annealed prior to usage, where said anneal significantlyreduces the fibers photosensitivity. In one embodiment the fiber is apure silica fiber without dopants in which it is difficult to writeBragg gratings even if it was H2 or D2 loaded without a subsequentanneal. In one embodiment the photosensitivity of the fiber pre and/orpost anneal is less than 50% of that of a typical fiber used forproducing fiber Bragg gratings, such as less than 25%, such as less than10%, such as less than 5%, such as less 1%, such as less than 0.1%, suchas less than 0.001%. In one embodiment the photosensitivity issubstantially zero.

In one embodiment water-band absorption (such as the peak absorptionpeak around 1400 nm) may be undesirable. In such an embodiment it may bepreferable to load the fiber with as little hydrogen as possible so thatin one embodiment the loaded fiber comprises bound deuterium relative tobound hydrogen (and/or their corresponding ions) of more than or equalto 1%, such as more than or equal to 10%, such as more than or equal to100%, such as more than or equal to 10,000% by atom. However, hydrogenmay be preferable for applications where such absorption is eitherinsignificant or even preferable, particularly when it is noted thathydrogen is commonly significantly cheaper than deuterium. Hydrogen mayalso be preferable for applications where the deuterium inducedOD-absorption around 1870 nm is undesirable.

In one embodiment the lifetime of a fiber has been found to increasewith increased ambient temperature during loading (see FIG. 7). Thisdependency indicates that providing energy during loading benefits thelifetime. In one embodiment, the extended lifetime depends on a chemicalprocess occurring between the material of the fiber andhydrogen/deuterium being loaded. In one embodiment, the lifetime hasbeen found to increase by introducing energy subsequent to loading, suchas by providing “subsequent irradiation” which is the term usedthroughout the application for the provision of energy to loadedmaterial during and/or subsequent to loading. In one embodimentsubsequent irradiation is taken to mean introducing energy at a ratehigher than provided by the general environment subsequent to loading,i.e. energy provided by keeping the fiber at room temperature or energyprovided by normal lighting is not subsequent irradiation. In oneembodiment this introduction of energy stimulates the chemical processbetween unbound Hydrogen/Deuterium and the material. In one embodimentit is therefore preferable that subsequent stimulation compriseslocalized contribution of energy. In one embodiment said chemicalprocess is binding of at least part of the loaded hydrogen and/ordeuterium to the material of the fiber. In one embodiment, the fiber isbeing loaded by subjecting it to hydrogen and/or deuterium under loadingconditions suitably to allow hydrogen and/or deuterium to bindchemically to said material(s). In one embodiment, said loadingcondition comprising at least one of a) raised temperature T, b) raisedpressure P, c) irradiation and d) subsequent irradiation. In oneembodiment the fiber is a silica fiber, so that in one embodiment saidchemical process is binding of at least part of the loaded Hydrogenand/or deuterium to the silica matrix, such as forming an OH⁻ or OD⁻bond. By providing such loading conditions a fiber comprising anincreased amount of bound hydrogen and/or deuterium is obtainable sothat in one embodiment the loaded material comprises more than 0.1 atompercent (at %) bound hydrogen and/or deuterium, such as more than 1 at%, such as more than 5 at %, such as more than 10 at %, such as morethan 20 atom percent, such as more than 50 at %.

In contrast to photosensitizing of fibers via loading the temperature Tis in one embodiment raised to allow for binding as discussed above, sothat T is more than or equal to 80° C., such as more than or equal to100° C., such as more than or equal to 120° C., such as more than orequal to 140° C., such as more than or equal to 160° C., such as morethan or equal to 180° C., such as more than or equal to 200° C., such asmore than or equal to 220° C., such as more than or equal to 240° C.,such as more than or equal to 260° C., such as more than or equal to280° C., such as more than or equal to 300° C., such as more than orequal to 350° C., such as more than or equal to 400° C., such as morethan or equal to 450° C., such as more than or equal to 500° C.

The fiber may and may not comprise a coating, such as a polymer coating.In an embodiment where the fiber comprises a polymer coating the loadingtemperature for loading deuterium and/or hydrogen is kept below themelting point of the polymer, such as below the softening temperature ofthe polymer. The practical upper limit for increasing the loadingtemperature is likely set by the coating of the fiber. In one embodimenthigh temperature coating may extend the possible deuterium loadingtemperature to above 250° C. or even higher. One example of a hightemperature coating is DeSolite® DF-0005 or DeSolite® DF-0009,manufactured by DSM Desotech Inc., which are designed for hightemperature fiber applications up to about 250° C.

Alternatively, fibers without coating may be produced, allowing evenhigher loading temperature e.g. up to and above 500° C. and/or loadingof the core (and optionally cladding) material may be performed prior orduring the process of forming the fiber i.e. prior to coating. In oneembodiment, the fiber is stripped of its coating and subsequentlyrecoated.

In one embodiment the chemical reaction time depends on the temperatureand/or pressure. In one embodiment the loading time during which thefiber is loaded is at least sufficient to ensure that thermalequilibrium has occurred. In one embodiment the loading time is lessthan 6 days, such as less than or equal to 4 days, such as less than orequal to 1 days, such as less than or equal to 1 day, such as less thanor equal to 12 hours, such as less than or equal to 6 hours, such asless than or equal to 3 hours, such as less than or equal to 2 hours,such as less than or equal to 1 hours, such as less than or equal to 30minutes, such as less than or equal to 1 minute.

An increased pressure of the loading gas may in one embodiment beutilized to provide a higher concentration of indiffused hydrogen and/ordeuterium in the fiber. Also, in one embodiment the pressure P is morethan or equal to 10 bars, such more than or equal to 25 bar, such morethan or equal to 50 bars, such more than or equal to 75 bar, such morethan or equal to 90 bar, such as more than or equal to 120 bar, such asmore than or equal to 160 bar, such as more than or equal to 200 bar,such as more than or equal to 500 bar, such as more than or equal to1000 bar, such as more than or equal to 2000 bar.

The above mentioned subsequent irradiation may in principle be anyirradiation suitable for providing energy to stimulate the chemicalprocess. In one embodiment said subsequent irradiation is light, such aslight introduced during use of the fiber. In one embodiment saidsubsequent irradiation alters the glass structure such as by creatingand/or activating defects which interact with hydrogen/deuterium.Further details of defects in glass may for example be found in thepaper “OPTICAL PROPERTIES AND STRUCTURE OF DEFECTS IN SILICA GLASS” byD. L. Griscom (Journal of the Ceramic Society of Japan. Vol. 99, no.1154, pp. 923-942, 1991). In one embodiment said irradiation is lightguided by the fiber. In one embodiment light otherwise applied in use ofthe fiber provides the activation energy. In use of the fiber, the pumplight, i.e. the light by which the fiber is pumped, entering the fibermay in one embodiment provide sufficient energy to allow unboundhydrogen/deuterium to interact with the fiber and/or create new defectswith which remaining unbound hydrogen/deuterium may interact. Inembodiments where the fiber is subjected to signal light and pump lightor signal light alone, pump light may also refer to signal light or pumpand signal light in combination. In the context of this application suchsubsequent irradiation of the fiber may be referred to as photoactivation. However, in principle any types of irradiation of the fiberproviding sufficient energy may be applied. In one embodiment UVradiation is applied as subsequent irradiation, since its higher photonenergy enables chemical reactions.

In one embodiment the fiber is cooled subsequent to loading and/orannealing in order to reduce diffusion out of the fiber prior tosubsequent irradiation. Accordingly, in one embodiment the fiber isstored at reduced temperature (cold storage) for a period in the timefrom production, i.e. loading, to subsequent irradiation. In oneembodiment, reduced temperature is less than −30° C., such as less than−40° C., such as less than −50° C., such as less than −60° C., such asless than −70° C., such as less than −90° C., such as less than −100° C.In one embodiment storing at reduced temperature preserves or limits theloss of deuterium and/or Hydrogen over time. In one embodiment storingat reduced temperature is applied substantially until the fiber is putin operation. In one embodiment cold storage is applied substantiallyuntil the fiber is subjected to subsequent irradiation. In oneembodiment the fiber is stored at reduced temperature after loadingeither before and/or after anneal. In one embodiment storing at reducedtemperature is applied at least part of the time between loading and/oranneal and subsequent irradiation and/or operation. In one embodimentthe fiber is subjected to storing at reduced temperature duringoperation.

In one embodiment the term subsequent in subsequent irradiation, e.g.photo activation, refers to a maximum time between loading andsubsequent irradiation less than or equal to 2 months, such as less thanor equal to 1 month, such as less than three weeks, such as less than orequal to 14 days, such as less than or equal to 7 days, such as lessthan or equal to 4 days, such as less than or equal to 1 day, such asless than or equal to 12 hours, such as less than or equal to 6 hours,such as less than or equal to 3 hours, such as less than or equal to 1hour, such as less than or equal to 1 minute. If the maximum timebetween loading and subsequent irradiation is exceeded, this will in oneembodiment lead to that the lifetime extension effect of the loading isdecreased relative to subjecting the fiber to irradiation immediatelyafter loading. In one embodiment storing the fiber at reducedtemperature significantly reduces the effects which cause this decreasein lifetime. In one embodiment the time where the fiber has been storedat reduced temperature is therefore not counted in the calculation ofthe maximum time as described above. In other words, maximum timebetween loading and subsequent irradiation refers in one embodiment to amaximum accumulated time, not counting time wherein said fiber has beenstored at reduced temperature, between loading and subsequentirradiation of less than 2 months, such as less than 1 month, such asless than 14 days, such as less than 7 days, such as less than 4 days,such as less than 1 day, such as less than 12 hours, such as less than 6hours, such as less than 3 hours, such as less than 1 hour, such as lessthan 1 minute. In one embodiment the term stored refers to the fiber notbeing used while in storage, whereas in one embodiment stored refers tothe fiber being in use while in storage.

In one embodiment subsequent irradiation is performed by irradiation ofthe fiber by applying irradiation transversely to the optical axis. Inone embodiment subsequent irradiation is performed by applyingirradiation to an end facet of the fiber, such as by coupling light orother radiation into the fiber. In one embodiment said irradiation isside coupled to the fiber. In one embodiment the fiber is subjected tosubsequent irradiation prior to annealing the fiber. In one suchembodiment the fiber ends of the fiber may be annealed to allow splicingof the fiber. In one embodiment said light is coupled into the fiber bysplicing the fiber to a light source of delivery fiber. In oneembodiment light is coupled into the fiber via free-space orbutt-coupling.

In one embodiment subsequent irradiation, e.g. photo activation,comprises pumping the fiber with light having a peak power densitywithin said fiber equal to or higher than 10 W/μm², such as equal to orhigher than 50 W/μm², such as equal to or higher than 100 W/μm², such asequal to or higher than 500 W/μm², such as equal to or higher than 1kW/μm², such as equal to or higher than 2.5 kW/μm², such as equal to orhigher than 5 kW/μm², such as equal to or higher than 10 kW/μm². In oneembodiment the subsequent irradiation comprises pumping said fiber withirradiation having an average power of 50 mW or more, such as 100 mW ormore, such as 500 mW or more, such as 1 W or more, such as more than 10W or more, such as 15 W or more, such as 20 W or more, such as 50 W ormore, such as 100 W or more. In one embodiment the subsequentirradiation has a duration of more than 1/1000 of a second, such as morethen 1/100 of a second, such as more than 1/10 of a second, such as morethan 1 second, such as more than 1 minute, such as more than 1 hour,such as more than 12 hours, such as more than 24 hours, such as morethan 48 hours, such as more than 1 week.

In principle the materials may be loaded at any time in the process offorming the fiber. However, consideration may have to be taken to ensurethat processes following the loading do not disrupt the achievedextension of the lifetime of the final fiber. Such a disruption mayoccur if a process following the loading comprises conditions whichallow the loaded hydrogen/deuterium to escape from the fiber e.g. asufficiently high temperature over a sufficiently long time.Accordingly, in one embodiment the loading of the core material, andoptionally of said cladding material, being performed prior to formingsaid fiber, during forming of said fiber and/or after forming saidfiber. Furthermore, as shown in FIG. 5 discussed below, fibers may, atleast partially, be regenerated so that in one embodiment the fiber isloaded after use.

After deuterium and/or hydrogen loading, the fiber is preferablyannealed. The anneal enables splicing the fiber to other fibers (plasmaheating of hydrogen/deuterium, such as in fusion splicing, may beexplosive) and reduces any added photosensitivity due to thesemolecules. However, as discussed below, an anneal may also serve otherfunctions such as providing increased lifetime of the fiber andincreased spectral stability of embodiments of supercontinuum lightsources according to the invention. Furthermore, in one embodiment thefiber is annealed prior to loading of the fiber, such as to relievedefects or stress in the fiber material. In one embodiment the fiber isanneal as a part of the drawing process. In one embodiment fiber isannealed between pulling the fiber and coating of the fiber. In oneembodiment the temperature of this anneal is equal to or less than thetemperature applied to the preform when pulling the fiber, such as 90%or less of that temperature, such as 80% or less of that temperature,such as 70% or less of that temperature, such as 60% or less of thattemperature, such as 50% or less of that temperature, such as 40% orless of that temperature, such as 30% or less of that temperature, suchas 20% or less that temperature, such as 10% or less that temperature.In one embodiment the anneal is a flash anneal providing a relativelybrief high energy irradiation.

In one embodiment excessive anneal temperature above approximate 1000°C. may lead to out diffusion of the bound hydrogen/deuterium and istherefore often undesirable. In one embodiment the anneal is performedfor more than 30 minutes, such as more than 1 hours, such as more than 4hours, such as more than 8 hours, such as more than 16 hours, such asmore than 32 hours, such as more 64 hours. In one embodiment the annealis a flash anneal of the fiber with or without coating. In oneembodiment said anneal is performed at a temperature T_(anneal) of morethan 40° C., such as more than 50° C., such as more than 60° C., such asmore than 70° C., such as more than 80° C., such as more than 90° C.,such as more than 100° C., such as more than 110° C., such as more than120° C., such as more than 130° C., such as more than 140° C., such asmore than 150° C., such as more than 160° C., such as more than 200° C.,such as more than 250° C., such as more than 300° C., such as more than350° C., such as more than 450° C., such as more than 550° C. In oneembodiment the anneal is performed in a chamber comprising an atmosphereadapted to be less aggressive with respect to the materials of thefiber, such as its coating, relative to atmospheric air. Surrounding thefiber with such an atmosphere during the anneal may provide a reductionin damage to the fiber and/or its coating due to the raised temperaturerelative to atmospheric air. In one embodiment said atmosphere of thechamber comprises a mixture of gasses that have low tendency to interactchemically with the optical fiber and/or its coating. In one embodimentthe anneal is performed in an atmosphere which is essentially free ofoxygen or other free radicals. In one embodiment the anneal is performedin an atmosphere substantially formed by Nitrogen, Argon, an inert gas,or a mixture thereof.

In one embodiment the anneal is performed in the same chamber that isused for loading. In one embodiment the fiber is subjected to subsequentirradiation during the anneal. In one embodiment the fiber is exposed toa flash irradiation in the chamber. In one embodiment the irradiation iscoupled into the fiber such as by irradiating the fiber end inside oroutside the chamber. In one embodiment a piece of transport fiber isspliced to the fiber to allow the entire fiber to reside in the chamber.In one embodiment the fiber end which the transport fiber is spliced tois annealed sufficiently to allow splicing. In one embodiment theirradiation is a general irradiation of at least part of the fibersurface such as via a light source in the chamber and/or through awindow in the chamber.

In one embodiment the lifetime of the fiber is extended relative to thelifetime of an otherwise identical fiber not subjected to loading bydeuterium and/or hydrogen by more than 50%,such as more than 100%, suchas more than 200%, such as more than 500%, such as more than 1000%, suchas more than 10,000%. The absolute lifetime of a fiber subjected tolight suitable for generating a supercontinuum may vary depending on theapplication as well as on the particular material of the fiber core. Inone embodiment the lifetime is more than 100 operating hours, such asmore than 200 operating hours, such as more than 2000 operating hours,such as more than 20000 operating hours, such as more than 50000operating hours.

In one embodiment the lifetime of a fiber has been found to scaleinversely with the peak power of the guided light to the fourth power.Accordingly, in such embodiment a reduction of the peak power by afactor of 2 may extend the lifetime of the fibre by a factor 16.

In one embodiment the lifetime is measured after an initial burn outtime of more than 1 minute, such as more than 1 hour, such as more than5 hours, such as more than 10 hours, such as more than 15 hours, such asmore than 20 hours, such as more than 24 hours, such as more than 2days, such as more than 7 days. In one embodiment, an initial burn outtime was observed to impose a decrease of 10% of the spectrum in thevisible emitted by a supercontinuum light source. In this context “burnout time” is understood as an initial time of operation where after thesystem/fiber/light source reaches a steady state operation. In thiscontext steady state is taken to mean that performance changes are slowrelative to the performance change during the burn out time, such as atleast 50% or lower, such as at least 100% slower, such as at least 300%slower. In one such example a laser has an output power which increases2% and thereafter reduces about 10% during the initial two days ofoperation after which it has a 2% reduction of output power during thenext 100 days of operation all else maintained equal. In such an example“burn out time” may be defined as the first two days of operation. Inone embodiment lifetime could be defined relative to the initial outputpower and in one embodiment relative to the maximum output power and inone embodiment relative to the output power after two days of operation.

In one embodiment the lifetime of a fiber is defined as time in whichthe fiber may be operated according to its operational purpose. In oneembodiment the lifetime of a fiber is defined as the time after whichthe absorption has increased by more than or equal to 1%, such as morethan or equal to 5%, such as more than or equal to 10%, such as morethan or equal to 20%, such as more than or equal to 30%, such as morethan or equal to 40%, such as more than or equal to 50%, such as morethan or equal to 100%, such as more than or equal to 200%, such as morethan or equal to 500%, such as more than or equal to 1000%. In oneembodiment said fiber forms part of a light source or another systememitting light. In one such embodiment the lifetime of the fiber isdefined as the time after which the optical output power of the lightsource or system is reduced by more than or equal to 1% all else equal,such as more than or equal to 5%, such as more than or equal to 10%,such as more than or equal to 20%, such as more than or equal to 30%,such as more than or equal to 40%, such as more than or equal to 50%,such as more than or equal to 60%, such as more than or equal to 70%,such as more than or equal to 80%, such as more than or equal to 90%. Inone embodiment the optical output of said light source and/or systememitting light is pulsed, in which case the optical output power may betaken to be the peak power, average power and/or pulse power. In oneembodiment the optical output power relates to the contribution of pumpenergy to the light source and/or system emitting light either viaoptical power (such as via a pump laser) or electrically or by othermeans. In one embodiment the lifetime is defined as more than or equalto a 1% increase in pump energy that is required to maintain the sameoptical output power, such as more than or equal to 5%, such as morethan or equal to 10%, such as more than or equal to 20%, such as morethan or equal to 30%, such as more than or equal to 40%, such as morethan or equal to 50%, such as more than or equal to 100%, such as morethan or equal to 200%, such as more than or equal to 500%, such as morethan or equal to 1000%. In one embodiment said absorption or reductionof optical power output increase is within more than or equal to 1% ofthe range of wavelengths for which the fiber/system/light source isoperated, such as more than or equal to 5%, such as more than or equalto 10%, such as more than or equal to 20%, such as more than or equal to30%, such as more than or equal to 40%, such as more than or equal to50%, such as more than or equal to 60%, such as more than or equal to70%, such as more than or equal to 80%, such as more than or equal to90%.

The amount of bound deuterium and/or hydrogen may in one embodiment bedetermined by spectroscopy. In one embodiment, such determinationcomprises determining the height of the absorption peak around 1380 nmof the OH⁻ bond and/or the absorption peak at 1870 nm of the OD⁻ bond.As the absorption cross sections of these bonds are known the measuredpeak height may in one embodiment be used to determine the concentrationof such bonds. In one embodiment said peak(s) may be applied todetermine an overrepresentation of hydrogen and/or deuterium relative toan unloaded fiber. In one embodiment, the amount of bound deuteriumand/or hydrogen may be determined by spectroscopy of the UV spectrumparticularly around 240 nm. As glass is highly absorbent in this regiononly a fairly short fiber may often be investigated at a time. Furtherdetails of such measurements may be found in M. J. Yuen, “Ultravioletabsorption studies of the germanium silicate glasses”, Appl. Opt., Vol.21, 1, 1982, pp. 136-140.

In one embodiment, an over representation of bound deuterium relative toan unloaded fiber, consistent with loading of the fiber according toinvention, may be determined at least partially by secondary ion massspectroscopy (SIMS). In one such embodiment the amount of OD⁻ isdetermined. In one embodiment the amount of SiD⁻ is determined as themeasurement chamber may comprise gaseous oxygen whereas there iscommonly little or no gaseous silicium in the measurement chamber. Inone embodiment the determination is performed relative to an unloadedfiber as variations in isotope concentration etc. may be cancelled.

In one embodiment determination of whether a fiber has been loadedaccording to the invention is performed by observing changes in thelifetime of the fiber pre and post an anneal designed to reduce bounddeuterium and/or hydrogen. In one embodiment the fiber is annealed at atemperature where the stability of the components of the fiber, such asthe coating, is ensured while the duration of the anneal may bedetermined by this temperature and the knowledge of the activationenergy for breaking the bond in question. A drop in lifetime due to thespecially designed anneal is consistent with a loading of the fiberaccording to the invention. Such a drop in lifetime may be a change inabsorption spectrum or a change in the emission spectrum for asupercontinuum light source (such as discussed below).

In one embodiment the optical fiber is a non-linear optical fiber. Inone embodiment a non-linear fiber is taken to mean a fiber which guideslight for at least a range of wavelengths λ_(min) to λ_(max) and has anon-linear parameter γ, wherein for at least part of said range theproduct γ·λ is more than or equal to 4·10⁻⁹ W⁻¹, such as more than orequal to 5·10⁻⁹ W⁻¹, such as more than or equal to 6·10⁻⁹ W⁻¹, such asmore than or equal to 7·10⁻⁹ W⁻¹, such as more than or equal to 8·10⁻⁹W⁻¹, such as more than or equal to 10·10⁻⁹ W⁻¹, such as more than orequal to 20·10⁻⁹ W⁻¹, such as more than or equal to 40·10⁻⁹ W⁻¹. Thenon-linear parameter γ is defined as

${\gamma = \frac{2\pi\mspace{31mu} n_{2}}{\lambda\mspace{25mu} A_{eff}}},$

where here n₂ is the nonlinear refractive index of the fiber materialand A_(eff) is the effective mode area of the fiber. As an example, n₂is commonly approximately 2.6·10⁻²⁰ m²/W for silica glass.

In one embodiment a non-linear fiber is taken to mean a fiber having anon-linear parameter γ when guiding a wavelength of 1550 nm wherein γ ismore than or equal to 3·10⁻³ (Wm)⁻¹, such as more than or equal to5·10⁻³ (Wm)⁻¹, such as more than or equal to 10·10⁻³ (Wm)⁻¹, such asmore than or equal to 15·10⁻³ (Wm)⁻¹, such as more than or equal to20·10⁻³ (Wm)⁻¹, such as more than or equal to 30·10⁻³ (Wm)⁻¹, such asmore than or equal to 40·10⁻³(Wm)⁻¹, such as more than or equal to50·10⁻³ (Wm)⁻¹.

In one embodiment a non-linear fiber is taken to mean a fiber having anon-linear parameter γ when guiding a wavelength of 1064 nm wherein γ ismore than or equal to 5·10⁻³ (Wm)⁻¹, such as more than or equal to10·10⁻³ (Wm)⁻¹, such as more than or equal to 15·10⁻³ (Wm)⁻¹, such asmore than or equal to 20·10⁻³ (Wm)⁻¹, such as more than or equal to30·10⁻³ (Wm)⁻¹, such as more than or equal to 40·10⁻³ (Wm)⁻¹, or such asmore than or equal to 50·10⁻³ (Wm)⁻¹.

In one embodiment a non-linear fiber is taken to mean a fiber where saidfiber guide light for at least a range of wavelengths λ_(min) to λ_(max)and a mode field diameter (MFD) of the fundamental mode least part ofsaid range the fraction MFD/λ is less than or equal to 5, such as lessthan or equal to 4, such as less than or equal to 3, such as less thanor equal to 2, such as less than or equal to 1.

In the above embodiments the range of wavelengths λ_(min) to λ_(max) maybe selected from the group of 350 nm to 2000 nm, 980 nm to 1550 nm, 1100to 1550 nm, 1300 nm to 1450 nm, 1500 nm to 4000 nm, 1500 nm to 6000 nm,1500 nm to 11000 nm. In one embodiment λ_(min) is more than or equal to300 nm, such as more than or equal to 350 nm, such as more than or equalto 450 nm, such as more than or equal to 600 nm, such as more than orequal to 750 nm, such as more than or equal to 900 nm, such as more thanor equal to 1050 nm, such as more than or equal to 1200 nm, such as morethan or equal to 1350 nm, such as more than or equal to 1500 nm, such asmore than or equal to 2000 nm, such as more than or equal to 3000 nm. Inone embodiment λ_(max) is less than or equal to 11000 nm, such as lessthan or equal to 10000 nm, such as less than or equal to 8000 nm, suchas less than or equal to 5000 nm, such as less than or equal to 2500 nm,such as less than or equal to 2000 nm, such as less than or equal to1500 nm, such as less than or equal to 1000 nm, such as less than orequal to 800 nm, such as less than or equal to 600 nm, such as less thanor equal to 500 nm. In one embodiment the range of wavelengths λ_(min)to λ_(max) is selected so as to limit the consideration to the range ofwavelengths wherein the fiber is single mode.

In one embodiment a non-linear fiber is taken to mean a fiber having anMFD when guiding a wavelength of 1550 nm wherein said MFD is less thanor equal to 10 μm, such as less than or equal to 8 μm, such as less thanor equal to 6 μm, such as less than or equal to 5 μm, such as less thanor equal to 4 μm, such as less than or equal to 3 μm, such as less thanor equal to 2 μm, such as less than or equal to 1 μm.

In one embodiment a non-linear fiber is taken to mean a fiber having anMFD when guiding a wavelength of 1064 nm wherein said MFD is less thanor equal to 6 μm, such as less than or equal to 5 μm, such as less thanor equal to 4 μm, such as less than or equal to 3 μm, such as less thanor equal to 2 μm, such as less than or equal to 1 μm.

In a preferred embodiment the fiber is a silica fiber, wherein at leasta part of the core being of silica or doped silica, preferably at leastthe entire core being of silica or doped silica, such as the entire coreand part or all of the cladding being of silica or doped silica.

In one embodiment the fiber is at least partly made of glass from thegroup of soft glasses, such as e.g. fluoride classes (such as e.g.ZBLAN, Fluorozirconate, Fluoroindate, Fluoroaluminate or Fluorogallate)or a chalcogenide (such as sulphide, selenide, and telluride and theIRT-SU product family offered by Corative) This embodiment may beparticularly suitable for applications where the wavelengths rangecomprises wavelengths larger than 2 μm. In one embodiment it might bepreferable to load with hydrogen instead of deuterium to avoid the ODabsorption peak around 1870 nm that may otherwise arise.

In one embodiment the invention relates to an optical system comprisingan optical fiber according to the invention and a feeding unit whereinsaid feeding unit is adapted to feed said fiber with light with a peakpower density within said fiber equal to or higher than 10 W/μm², suchas equal to or higher than 50 W/μm², such as equal to or higher than 100W/μm², such as equal to or higher than 500 W/μm², such as equal to orhigher than 1000 W/μm², such as equal to or higher than 2500 W/μm², suchas equal to or higher than 5,000 W/μm², such as equal to or higher than10,000 W/μm². Application where such high peak power density in thefiber are in the present document referred to as high powerapplications. It should be noted, that while reference is made to peakpower which commonly is a feature of pulsed light, peak power may inthis context refer to CW light as well.

In one embodiment said feeding unit comprises a pump light source and inone embodiment the feeding unit also comprises one or more amplifiers.In principle the feeding unit may be any optical system feeding light tothe fiber.

As the fiber according to the invention has no or reduced degradationdue to exposure to high peak power such a system may have an extendedlifetime of operation depending on the significance of the fiberperformance and life-time relative to that of other components in thesystem.

In one aspect the invention relates to a method of producing an opticalfiber comprising a core and a cladding comprising a core material and acladding material, respectively, said fiber having extended lifetime inhigh power applications, the method comprising

-   -   a. loading said core material and optionally said cladding        material with hydrogen and/or deuterium    -   b. optionally annealing said for a time t_(anneal) at a        temperature T_(anneal).

Such a method may be advantageously applied to produce a fiber accordingto the invention and any features described in relation to features ofthe fiber may apply mutatis mutandis to the method of producing thefiber.

In one embodiment the said loading is performed by subjecting the corematerial and/or the cladding material to hydrogen and/or deuterium underloading conditions suitable to allow hydrogen and/or deuterium to bindchemically to said material(s). In one embodiment the said loading isperformed by subjecting the core material and/or the cladding materialto hydrogen and/or deuterium under loading conditions comprising atleast one of a) a raised temperature T, b) a raised pressure P c)irradiation and/or d) subsequent irradiation.

In an embodiment the invention relates to an apparatus comprising anoptical fiber according to the invention, an optical system according tothe invention, a light source according to the invention and/or a fiberproduced according to the invention. In one embodiment the apparatusconstitutes a system for performing one from the group of laserprecision spectroscopy, various forms of fluorescent microscopy, guidingof surgical and/or therapeutic light, astronomy (guide star generation),confocal microscopy endoscopy, optical coherence tomography (OCT) andcombinations thereof.

As mentioned above, it has been found that it is often possible toregenerate a degenerated fiber, thus providing a fiber with extendedlifetime compared to an identical fiber not subjected loading.Accordingly, in one embodiment the invention relates to a method ofregenerating a fiber comprising a core and a cladding comprising a corematerial and a cladding material, respectively, said fiber havingincreased absorption in the visible due to subjection to light in a highpower application the method comprising loading the fiber with hydrogenand/or deuterium. In one embodiment loading of said fiber is carried outunder loading conditions allowing hydrogen and/or deuterium to bind tothe core material and/or a cladding material. In one embodiment loadingof said fiber is carried out under loading conditions comprising atleast one of a) a raised temperature T, b) a raised pressure P c)irradiation and/or d) subsequent irradiation. Furthermore, any featuresdescribed in relation to loading of the fiber prior to use discussedabove may in one embodiment apply mutatis mutandis to the method ofproducing the fiber.

It should be emphasized that the term “comprises/comprising” when usedin this specification is taken to specify the presence of statedfeatures, integers, steps or components but does not preclude thepresence or addition of one or more other stated features, integers,steps, components or groups thereof.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained more fully below in connection withpreferred embodiments and with reference to the drawings in which:

FIG. 1 shows a part of a typical supercontinuum spectra in initialoperation of a prior art microstructured optical fiber (A) and after 35hours of operation (B) all else equal. The reduction in the visiblespectrum testifies to the degradation of the fiber;,

FIG. 2 shows measured attenuation for a prior art microstructurednonlinear fiber operated for 35 hours as a function of the position ofthe fiber;

FIG. 3 shows 633 nm absorption as function of position in themicrostructured nonlinear fiber, where position means length from theentrance of the pump light;

FIG. 4 shows the initial visible supercontinuum spectra (A), after 35hours of operation where the visible dip is observed (B) and again afterheating the fiber to 250° C. (C);

FIG. 5 shows supercontinuum spectra after 35 hours of operation (A)where a visible dip is observed and again after heating the fiber to250° C. (B) and after the fiber has been deuterium loaded and annealed(C);

FIG. 6 shows measured visible power as function of time formicrostructured nonlinear fibers deuterium loaded at 160 C (A), at 80 C(B) and not deuterium loaded (C);

FIG. 7 shows extracted lifetime as function of three different deuteriumloading temperatures (A) and an exponential fit to the measurements (B);

FIG. 8 shows measured spectra for a deuterium loaded microstructurednonlinear fiber after 0 hours (A), 188 hours (B), 260 hours (C), 305hours (D) and 450 hours (E) of operation;

FIG. 9 shows measured visible power as function of time formicrostructured nonlinear fibers with less glass impurities compared tothe previous figures;

FIG. 10 shows results from a transmission experiment involving amicrostructed fibre, showing the transmission just post loading (C), and22 hours after loading where the ends of the fiber has been sealed (B).For comparison, a transmission curve is shown for a similarly loadedmicrostructured with unsealed fiber ends 2.5 hours post loading (A);

FIG. 11 shows one example of spectra obtained from a supercontinuumlight source comprising a non-linear microstructured fibre according tothe invention. The spectra are an initial spectrum (A) and a spectrumafter a 160 hour operation (B);

FIG. 12 shows the coupling efficiency spectra from a supercontinuumlight source according to the invention to a single mode fiber. Thespectra are an initial spectrum (A), and spectra after a 273 hours (B)and 535 hours of operation (C). The microstructured fiber has beenannealed for 4 hours at 80 C at a pressure of a standard atmosphere;

FIG. 13 shows M² measurements vs. wavelength for a supercontinuum lightsource according to the invention. The microstructured fiber has beenannealed for 4 hours at 160 C in an atmosphere of nitrogen. The spectraare an initial spectrum (A) and a spectrum after 3500 hours of operationwhere the fiber has degraded in the visible region (B);

FIG. 14 shows spectra obtained from a supercontinuum source according tothe invention after 3200 hours of operation. The microstructured fiberhas been annealed for 4 hours at 160 C in an atmosphere of nitrogen. Thespectra A is for a supercontinuum source with an uncoiled loadednonlinear fiber according to invention, whereas in B, the nonlinearfiber is coiled with a radius of 25 mm; and

FIG. 15 is a schematic cross-sectional view of an optical fiberaccording to an embodiment of the invention. The drawing is not made toscale, and is not intended to illustrate the relative diameters andthicknesses of the core material and cladding.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, and should not be taken to limit theinventions as set forth by the appended set of claims.

DETAILS OF THE INVENTION

In the following some examples will comprise discussion of the inventionbased on measured data. The conclusions drawn from these should notnecessarily be considered limited to the specifics of the underlyingexperiments.

Super Continuum Generation in Microstructured Fibers

In one embodiment, of the invention the optical fiber is amicrostructured optical fiber. Microstructured optical fibers are arelatively new technical field where the properties of the waveguide maybe designed with a relatively large degree of freedom. Such fibers arecommonly made of pure silica comprising a pattern, often made of holesor doped glass, extending in the longitudinal direction of the fiber.The freedom of design makes such fibers interesting for applicationrequiring specific non-linear properties of the fiber. One suchapplication is supercontinuum generation wherein a fiber based source iscable of generating a wide spectral output. Supercontinuum (SC)generation in microstructured fibers has been studied for several yearsas a source of broadband light (termed white light or supercontinuum).While new applications of such sources are continuously discovered,several have already been identified, such as various forms offluorescent microscopy, laser precision spectroscopy, and opticalcoherence tomography (OCT). High brightness emission in the visible partof the spectrum is especially important for confocal fluorescentmicroscopy. SC-generation with relatively high power in the visible hasbeen targeted in the experiments presented here. Most research has sofar been based on seeding the microstructured fiber with femtosecond(fs)-lasers but SC-generation using nanosecond- and picosecond(ps)-lasers has also been demonstrated.

As microstructured fibers often guides by holes extending in thecladding such fibers often consist entirely of un-doped silica (i.e.both core and cladding are made of silica) in opposition to e.g.standard single mode communication fibers where the core is commonlydoped with germanium in order to change the refractive index.Accordingly, in one embodiment the core of the fiber comprises aGermanium content of less than or equal to 20 at %, such as less than 5at %, such as less than 3 at %, such as less than 2 at %, such as lessthan 0.1 at %, such as less than 0.01 at %, such as less than 0.001 at%.

In one embodiment, submitting the fiber to subsequent irradiation maysignificantly improve the lifetime of the fiber. In one embodimentexperimental results have shown a reduction of optical power of asupercontinuum light source of 30% after 40 hours of operation when thefiber has not been loaded, 30% after 80 hours when the fiber has beenloaded and the ends subsequently sealed and the fiber stored at roomtemperature for over 1 month and only 4% after 200 hours when the fiberwas loaded and photo activated within 1 week of load with the endssealed. It may be noted that in one embodiment experiments have shownthat the above degradation scales with applied peak power to the fourthpower. Accordingly, the above periods may in one embodiment be extendedby e.g. a factor of 16 by reducing the applied optical peak power with50%. In one embodiment the latter result was found to drop approximately10% over the first 24 hours where after the source showed thisstability.

In one embodiment, the application of irradiation of the fibersubsequent to loading utilizes gaseous hydrogen/deuterium residing inthe microstructures formed by holes of a microstructured fiber. In oneembodiment the fiber is annealed to improve the life time of the fiberand/or to allow splicing of the end, such as discussed above. However inone embodiment it may be preferable to allow gaseous hydrogen/deuteriumto reside in the holes of the microstructure just prior to or duringsubsequent irradiation, such as photo activation. In such embodiments itmay be preferable to seal one or both ends of the fiber. Said sealing isperformed prior to storage at reduced temperature and/or prior toloading. In one embodiment sealing is performed by supplying sufficientthermal power, such as by an arch or fusion splicer. In one embodimentsealing is performed by applying a resin, such as epoxy to the fiberends. In one embodiment said resin may be UV cured. As shown in FIG. 10,the initial absorption shown by the dip in transmission around 1710 nm(101) is substantially unchanged for a fiber just after loading (B) and22 hours post loading with sealed ends (C). On the other hand the samepeak is shown to be substantially gone after just 2.5 hours when theends are left unsealed (A). In some embodiments of the invention theapplication of such microstructured fibers is selected from the group ofsupercontinuum generation, optical power transport, opticalamplification and forming part of a laser cavity. It is preferred thatthe irradiation is performed subsequent to loading; however, in oneembodiment, it is performed during loading.

In one embodiment sealing of the fiber ends may be preferable tominimize the entrance of impurities into the fiber via themicrostructures. In one embodiment such impurities comprises water,which may condensate on the fiber post having the fiber stored atreduced temperature. In one such embodiment said water may otherwisetravel to the inside of the fiber via capillary forces and/or diffusion.

As will be apparent to the skilled person, the considerations in regardto gaseous hydrogen/deuterium in the microstructures and the sealing ofthese may in one embodiment be relevant for applications ofmicrostructured fibers other than super continuum generation, such aspower delivery.

In one embodiment the invention relates to a supercontinuum light sourcecomprising a pulsed pump light source and an optical fiber according tothe invention wherein said pump source is adapted to provide light witha peak power density within said fiber equal to or higher than 10 W/μm²,such as equal to or higher than 50 W/μm², such as equal to or higherthan 100 W/μm², such as equal to or higher than 0.5 kW/μm², such asequal to or higher than 1 kW/μm², such as equal to or higher than 2.5kW/μm², such as equal to or higher than 5 kW/μm², such as equal to orhigher than 10 kW/μm², and/or wherein said pump and fiber is adapted toprovide an output spanning over at least one octave with at least 10μW/nm and/or wherein said pump and said fiber is adapted to provide amaximum modulation instability gain Ω_(max) such as larger than 20, suchas larger than 40.

Here the modulation instability gain Ω_(max) is given by

${\Omega_{\max} = {\pm \sqrt{\frac{2\gamma\; P_{peak}}{\beta_{2}}}}},$

where β₂ is the group velocity at the pump wavelength, P_(peak) is thepeak power of the pump and γ is the pump wavelength.

In one example more than one octave span has been achieved with themicrostructured nonlinear fiber SC-5.0-1040 from the Danish companyCrystal Fiber A/S. Using this fiber with a peak power of 200 W pumped at1064 nm provided Ω_(max)=22 (A peak power of 200 W is e.g. obtainedthrough a 50 MHz, 100 mW input signal with 10 ps pulses).

In one embodiment the pump light source comprises a laser which may bepulsed or continuous wave. The laser may in principle be any suitablelaser to provide the desired wavelength(s), power and/or temporalperformance (i.e. pulse length, repetition rate etc.). In one embodimentsaid laser is a fiber laser, such as a mode locked fiber laser. In oneembodiment the pump light source further comprises one or moreamplifiers arranged to amplify the output of said laser. In one suchembodiment the laser light source is formed by a so-called MOPAconfiguration.

The phrase spanning over at least one octave with at least a specificpower value (per nm wavelength) is in this context of the presentinvention taken to mean that the optical spectrum of the output of thelight source spans at least an octave defining the outer limits of saidspectrum by said specific power value. The spectrum may have holes;however, it is assumed that more than 25% of the spanned spectrum has atleast the specific power value. In an embodiment at least 30% of thespanned spectrum has at least the specific power value, such as at least40%, such as at least 60%, such as at least 80%, such as at least 99%,such as at least 99.9%.

In one embodiment, the output spans over at least one octave with atleast 50 μW/nm, such as more than or equal to 500 μW/nm, such as morethan or equal to 1 mW/nm, such as more than or equal to 5 mW/nm, such asmore than or equal to 10 mW/nm. Depending on the chosen power limit oneembodiment may also span over more than or equal to 0.5 octave, such asmore than or equal to 1.5 octave, such as more than or equal to 2octaves.

In one embodiment the spectral degradation of a supercontinuum lightsource light comprising a non-linear microstructured fiber is less than5% over more than 50 hours, such as over more than 100 hours, such asover more than 500 hours, such as over more than 1000 hours. In oneembodiment the light system with which the light source is made tointeract is recalibrated at least every 1000 hours, such as at leastevery 500 hours, such as at least every 100 hours, such as at leastevery 50 hours. In one embodiment anneal of the fiber is preferred inorder to improve the spectral stability.

In one embodiment the non-linear fiber is polarization maintaining (PM)as this in one embodiment may provide a similar spectrum with a 50%reduction of the necessary peak power. In one embodiment the degradationscales with the applied peak to the fourth power so that a significantextension of the lifetime of the fiber and thereby the light source maybe available. In one embodiment this requires a good polarizationextinction ratio of the feed system pumping the non-linear fiber, suchas more than 10 dB, such as more than 13 dB, such as more than 15 dB,such as more than 17 dB, such as more than 20 dB.

In one embodiment the non-linear fiber and the feed system, i.e. feedingunit, are coupled using bulk optics. However, as a bulk optic couplingsystem may provide many degrees of freedom and is often prone tomechanical and thermal instability splicing of the two components may bepreferred. In one embodiment the feed system is spliced to thenon-linear fiber. In one embodiment the feed system comprises an opticalamplifier providing the output of the feed system into the non-linearfiber. In one embodiment there is a significant mismatch in core sizebetween the feed system (e.g. a diameter of 11 μm) and the non-linearfiber (e.g. a diameter of 3 μm). In one embodiment this mismatch isreduced by allowing the core of the non-linear fiber to expand duringsplicing.

In the following measured data were obtained for a supercontinuum lightsource comprising a pump source and a non-linear microstructured silicafiber. The fiber was pumped at 1064 nm with 8 ps pulses at a repetitionrate of 80 MHz providing a 15 W input average power (23 kW peak power).The fiber had a mode field diameter of 3.5 μm, and air-filling fractionof about 50% and was approximately 7 meters in length. The pump lightsource was formed by a master-oscillator power amplifier (MOPA) designcomprising a mode-locked laser, a preamplifier and a power amplifierfollowed by two pre-amplifiers.

The length of the fiber is preferably kept short to keep the consumptionof fiber to a minimum while still providing sufficient length to allowthe non-linear processes underlying a supercontinuum to provide adesirable spectrum. This length commonly depends on the shape of thepulses as shorter fiber is commonly sufficient for shorter pulses. Inone embodiment the non-linear fiber have a length of 1 cm or longer,such 10 cm or longer, such 1 m or longer, such 5 m or longer, such as 8m or longer, such as 10 m or longer.

In one embodiment the non-linear microstructured fiber is 500 m or less,100 m or less, 50 m or less, such as 30 m or less, such as 10 m or less.

FIG. 1 shows typical supercontinuum spectra in initial operation of aprior art microstructured optical fiber (A) and after 35 hours ofoperation (B) all else equal. The reduction in the visible portion ofthe spectrum extending from about 450 nm to about 750 nm testifies tothe degradation of the fiber. The phenomenon is investigated further bythe measurements shown in FIG. 2 showing attenuation for the prior artmicrostructured nonlinear fiber operated for 35 hours as a function ofthe position of the fiber from the entrance of the pump light. A ismeasured through the first 3 m of the nonlinear fiber (NL-fiber), B isthrough 3-4 m, C through 4-5 m and D through 5-7 m. The curves areobtained by subtracting a spectrum obtained with a 7 m long referencenon-linear fiber which has not been operated with high power for alonger duration of time. Very large absorption is observed in thevisible part of the spectrum due to the degradation of the fiber. Thedip at 0.9 μm and 1.4 μm likely stems from the single-mode cut-off forthe microstructured nonlinear fiber and differences in O—H peakabsorption for the microstructured nonlinear fiber and the referencefiber, respectively. If the degradation is caused by interaction withthe relatively high peak powered pump pulse, then the degradation isexpected to be larger closer to the pump laser where the peak power ismaximal. As the pump pulses travel through the fiber their average powerdecreases due to attenuation. Furthermore, non-linear effects will tendto broaden the pulse to reduce the peak power of the pulses along thefiber. Therefore less degradation is expected along the fiber furtherfrom the injection of the pump pulses. This tendency is seen in thisexample as the absorption drops as the fiber sections are taken fromparts which were operated further and further from the pump. This trendis also found in FIG. 3 showing that measurements of the absorption at633 nm as a function of distance from the entrance of the pump light fitwell to an exponential.

FIG. 4 shows the supercontinuum spectra in the beginning of theexperiment (A), after 35 hours were the visible dip is observed (B) andagain after heating the fiber to 250° C. (C).

The heating seems to partly regenerate the fiber. The inventorshypothesize that the regeneration of the fiber may be an indicator ofthe pump light altering the structure of at least a part of the glass.Allowing the glass to reach a higher temperature may allow the glass toresettle causing it to at least partly regenerate.

FIG. 5 shows supercontinuum spectra after 35 hours (A) where a visibledip is observed and again after heating the fiber to 250° C. (B) andafter the fiber has been deuterium loaded and subsequently annealed (C).The deuterium loading clearly regenerated the fiber and the spectrumresembles the initial spectrum (see FIG. 4) without any visible dip inthe spectrum.

FIG. 6 shows measured visible power of a supercontinuum source asfunction of time for 3 pieces of identical microstructured nonlinearfibers deuterium loaded at 160 C (A), at 80 C (B) and not deuteriumloaded (C). The lifetime of the deuterium loaded fibers is increased byat least 2 orders of magnitude compared to unloaded fibers. All fibersare loaded at 100 bar pressure with 100% deuterium.

FIG. 7 shows lifetime extracted from FIG. 6 as a function of deuteriumloading temperature (A) and an exponential fit to the measurements (B).In this example the lifetime was defined as the time where the visiblepower has decreased 30%. Depending on the application the lifetime maybe defined as where the visible power has decreased by more than 40%,such as more than 50%, such as more than 70%, such as more than 80%,such as more than 90%. Visible light may in the context be defined as anintegral of light in the range 0.4 to 0.7 μm. Alternatively, one or morewavelength values may be specified such as the power at 650 nm and/or at633 nm. As discussed above these results may indicate that the lifetimeof the fiber increases exponentially with loading temperature, at leastfor the temperatures applied here and that loading at increasedtemperature may be advantageous as long as practical factors such as thetemperature tolerance of the coating is considered.

FIG. 8 shows measured spectra for a deuterium loaded microstructurednonlinear fiber after 0 hours (A), 188 hours (B), 260 hours (C), 305hours (D), and 450 hours (E) . The prominent broad dip for thenon-loaded microstructured fiber in the visible spectrum from 0.4 to 0.7μm is no longer observed. In addition to increasing the lifetime of themicrostructured nonlinear fiber the deuterium loading has also shown inthis embodiment to significantly alter the spectral changes of the fiberunder operation compared to unloaded fibers. Relative to an unloadedfiber the degradation is no longer observed as a dip in the visiblespectrum, but as a broadening of the long wavelength peak around 475 nmand a slow overall decrease of visible power.

FIG. 9 shows measured visible power as function of time formicrostructured nonlinear fibers with less glass impurities for aDeuterium loaded (A) and unloaded (B) fiber. Again the lifetime of thedeuterium loaded fiber (A) is significantly increased compared to theunloaded fiber (B). The increase in output power for the deuteriumloaded fiber after 750 hours is due to an increase in pump power.Compared to FIG. 5 the lifetime is significantly extended indicatingthat the lifetime may also depend on the glass impurity level.

FIG. 11 shows one example of spectra obtained from a supercontinuumlight source comprising a non-linear microstructured fibre according tothe invention. The shown spectra are an initial spectrum (A) and aspectrum after a 160 hours of operation (B). It has been observed thatin many of the embodiments of the invention the spectrum shows areduction in a peak at short wavelengths, commonly around 480 nm,whereas a peak rises from the remaining spectrum around 550 nm in thepresent embodiment. This peak around 550 nm has not been observed forsupercontinuum light sources comprising an unloaded non-linearmicrostructured fiber as evident from the other figures.

In one embodiment of a supercontinuum light source according to theinvention it has been observed that the increase in output power around550 nm shown in FIG. 11 occurs simultaneously with a decrease in spatialmode quality of the output light at wavelengths lower than about 550 nm,i.e. the light is increasingly multimoded. This decreased beam qualitymay be identified in many ways, e.g. by measuring the couplingefficiency to a single-mode fiber or by measuring the M-square value.FIG. 12 shows the coupling efficiency from a super continuum sourceaccording to the invention to a single mode fiber as a function ofwavelength for 0, 273 and 535 hours of operation. The measurementuncertainty is a few percent and thus the coupling efficiency above 550nm is within the measurement uncertainty unchanged with time. However,below 550 nm the coupling efficiency drops with time.

FIG. 13 shows the M² spectra for a supercontinuum source according tothe invention. The spectra are an initial spectrum (A) and a spectrumafter 3500 hours of operation where the fiber has degraded in thevisible region (B). Notice that the time until degration occurs is muchlonger than in FIG. 12. The difference between FIGS. 13 and 12 is thatin FIG. 13 the nonlinear fiber has been annealed for 4 hours at 160 C inan atmosphere of nitrogen prior to use, whereas in FIG. 12 the nonlinearfiber has been annealed for 4 hours at 80 C in a standard atmosphereprior to use.

In one embodiment the time of operation until the described changearound 550 nm occurs depends on the anneal conditions of the fiber usedto generate the super continuum, such as a microstructured or standardnonlinear fiber. In one embodiment increased temperature of the annealextends this time of operation before such changes around 550 nm areobserved. In one embodiment the time during which the fiber is annealedhas a similar effect. In one embodiment this correlation indicates thattoo much residual hydrogen or deuterium in the fiber may affect saidtime of operation. In one embodiment the fiber is annealed aftersubsequent irradiation. In one embodiment this has the effect ofallowing residual hydrogen and/or deuterium to provide a benefit duringsubsequent irradiation and subsequently to that removing at least partof the residual hydrogen/deuterium. In one embodiment the fiber isannealed prior and post subsequent irradiation.

As the light below about 550 nm becomes more multimoded more opticalenergy may be coupled to these higher order modes. In one embodiment thefiber is coiled to strip these higher order modes for wavelengths ofless than about 550 nm and/or to prevent coupling to such higher ordermodes. In one embodiment the fiber comprises a chirally coupled core tostrip these higher order modes for wavelengths of less than about 550 nmand/or to prevent coupling to such higher order modes. FIG. 14 shows theoutput spectrum of a supercontinuum light source according to theinvention after 3200 hours of operation. The microstructure fiber hasbeen annealed for 4 hours at 160 C in an atmosphere of nitrogen prior touse. The spectra A is for a normal supercontinuum source, whereas in Bthe nonlinear fiber is coiled with a radius of 20 mm. It is observedthat the coiling decreases the output power at the 550 nm peak andfurthermore increases the short wavelength peak around 480 nm as well asrestores the M² value.

The inventors have surprisingly found that in one embodiment arelatively narrow coil is required but also that such a coil may in oneembodiment function even when the fiber is a micro structured fiber,which would otherwise be considered sensitive to such mechanical stress.In one embodiment at least part of the fiber is coiled with a minimumdiameter R where R is less than or equal to 40 mm, such as less than orequal to 30 mm such as less than or equal to 25 mm, such as less than orequal to 20 mm, such as less than or equal to 15 mm, such as less thanor equal to 10 mm, such as less than or equal to 5 mm. In a coil theradius of each winding may vary e.g. depending on the winding method andwhether the winding is the inner most winding or not. In this case, theminimum radius R refers to the radius of the winding with the smallestradius.

In one embodiment the position of the coil relative to where the pumplight is injected affects the efficiency of the coil to prevent the dropin average optical output power). Closer to the injection of the pumplight the peak power of the pump pulse are higher and the formation ofthe different wavelengths of the output spectrum of the super continuumsource may in one embodiment be at least partly related to the positionalong the fiber. Accordingly, in one embodiment the fiber has an inputend coupled to said pump light source and an output end, wherein saidcoiling is performed less than 50 cm from the input end, such as lessthan 40 cm, such as less than 30 cm, such as less than 20 cm, such asless than 10 cm, such as less than 5 cm. To use the stripping effectprovided by the coil at a position where specific wavelengths of theoutput spectrum are generated, the coil may, in one embodiment, cover atleast a distance which is more than 5 cm from the input end, such asmore than 10 cm from the input end, such as more than 20 cm from theinput end, such as more than 30 cm from the input end, such as more than40 cm from the input end, such as more than 50 cm from the input end,such as more than 70 cm from the input end, such as more than 80 cm fromthe input end. In one embodiment little of the fiber is required to becoiled to reduce the said degradation at wavelengths below 550 nm. Inone embodiment more of the fiber is required to be coiled, such as toprevent the degradation occurring in an uncoiled section of the fiber.In one embodiment more than or equal to 10% of said fiber is coiled,such as more than or equal to 20% of said fiber is coiled, such as morethan or equal to 30% of said fiber is coiled, such as more than or equalto 40% of said fiber is coiled, such as more than or equal to 50% ofsaid fiber is coiled, such as more than or equal to 60% of said fiber iscoiled such as more than or equal to 70% of said fiber is coiled, suchas more than or equal to 80% of said fiber is coiled, such as more thanor equal to 90% of said fiber is coiled, such as 100% of the fiber iscoiled. In one embodiment one winding is sufficient to prevent thedegradation discussed above and/or to strip higher order modes and/orsuppress coupling of light from the fundamental mode to higher ordermodes. However, in one embodiment two or more windings may be requiredto provide sufficient effect. Accordingly, in one embodiment said coilcomprises 1 or more windings, such as 2 or more windings, such as 10 ormore windings, such as 25 or more windings, such as 50 or more windings,such as 100 or more windings. In one embodiment the number of requiredwinding decreases with the winding radius. In one embodiment it may bepreferable to wind a long section, such as all, of the fiber with alesser larger radius rather than submit the fiber to the mechanicalstress imposed by a smaller radius.

As will be obvious to the skilled person, the detailed effects ofincreasing and decreasing peaks as well as the region of wavelengths forwhich the beam quality drops discussed here are exemplary relating tothe embodiments. It is clear that one or more of the discussed effectsmay depend on the overall design of the supercontinuum light source suchas pump properties (e.g. wavelength, peak power, pulse energy etc.)and/or fiber (e.g. material, core size, dopants, mode field diameteretc.). Therefore, it should be recognized that increased annealtemperature and/or stripping of higher order modes, such as describedabove, may have application in any of the embodiments of the inventionincluding the optical fiber as such and making of the same.

In one embodiment lifetime extension is provided by bounddeuterium/hydrogen relative to total number of impurities and/or defectsin the core and in some application also in the cladding material.Accordingly, in one embodiment the core of the fiber being a solid core(preferably silica) wherein the fraction of bound hydrogen and/ordeuterium relative to the total number of impurities and/or defects ismore than or equal to 10%, such as more than or equal to 20%, such asmore than or equal to 30%, such as more than or equal to 40%, such asmore than or equal to 50%, such as more than or equal to 60%, such asmore than or equal to 70%, such as more than or equal to 80%, such asmore than or equal to 90%, such as more than or equal to 99%, such asmore than or equal 99.9%. In this context all compounds in the glassapart from SiO₂ are considered impurities.

The invention is defined by the features of the independent claim(s).Preferred embodiments are defined in the dependent claims. Any referencenumerals in the claims are intended to be non-limiting for their scope.

Some preferred embodiments have been shown in the foregoing, but itshould be stressed that the invention is not limited to these, but maybe embodied in other ways within the subject-matter defined in thefollowing claims.

Super Continuum Generation in Standard Fibers

In the present context the term standard fiber refers to fibers guidinglight in a solid material substantially free of microstructures. Guidingis obtained by total internal reflection at the interface between a coreof the fiber and a cladding, so that the refractive index of the core ishigher than the refractive index of the cladding. The fiber may comprisemultiple cores and/or claddings such as in double clad fibers or pandafibers.

Similarly to microstructured fibers, supercontinuum (SC) generation maybe possible in standard fibers provided that suitable nonlinear anddispersion properties are provided.

In one embodiment SC generation is obtained from standard fiber such asa conventional single-mode fiber similar to what was demonstrated byWatt et al., Generation of supercontinuum radiation in conventionalsingle-mode fibre and its application to broadband spectroscopy, Appl.Phys. B 90, 47-53 (2008). Here a single mode fiber (SMF28, Corning) ispumped at 1064 nm with 5 ps pulses. In one embodiment an unloaded SMF28fiber will experience similar degradation to that observed in amicrostructured fiber, and this degradation may be prevented, reducedand/or repaired by way of the invention. In one embodiment, increasedload time relative to that of a similar microstructured fiber will bebeneficial as a standard fiber lacks holes and therefore comprises morematerial in and/or around the core.

In one embodiment, guiding in the standard fiber is obtained by dopingthe cladding in order to reduce the refractive index, such as byfluoride. In one embodiment guiding of the standard fiber occurs inundoped silica.

In one embodiment SC is obtained, at least in part, from tapering of thefiber, similarly to what was demonstrated by Lu et al., Generation ofbroadband continuum with high spectral coherence in tapered single-modeoptical fibers, Opt. Expr., Vol. 12, No. 2 (2004). Here 100 fs pulseswere pumped into the fiber at wavelengths ranging from 780 nm to 920 nm.In one embodiment, a tapered standard fiber will experience similardegradation to that observed in a microstructured fiber and that thisdegradation may be prevented, reduced and/or repaired by way of theinvention.

Optical Power Transport

Optical fibers (standard and microstructured) may be applied totransport optical energy (CW and/or pulsed) in applications such asguiding of surgical and/or therapeutic light, optical sensing, materialsprocessing and measuring technology. Accordingly, in one embodiment anoptical fiber is arranged to receive light from a feeding unit andtransport said light substantially unchanged. In this contextsubstantially unchanged is taken to mean that the output of the fibermay be substantially calculated by multiplying with a linear transferfunction. Substantially calculated is taken to mean that the calculationis accurate within less than 20% deviation, such as less than 10%deviation, such as less than 5% deviation, such as less than 1%deviation. In one embodiment such an optical fiber will experiencesimilar degradation to that observed in a microstructured fiberdescribed above, and that this degradation may be prevented, reducedand/or repaired by way of the invention.

Active Fibers

In one embodiment the fiber according to the invention is an activefiber suitable for providing optical amplification when pumped withexcitation light also referred to as pump light. In one embodiment saidactive fiber forms part of an optical amplifier and in one embodimentsaid active fiber forms part of a laser, such as forms part of a lasercavity.

In one embodiment a fiber according to the invention forms part of alaser cavity. Even though a laser may operate at a wavelength far fromwhere deterioration is commonly observed (e.g. in the visible range) atail of absorption may still be present at the operating wavelength. Ina laser light may pass the fiber a high number of times before beingcoupled out. For this, and other reasons, even a small deterioration mayaffect the performance of the laser. In one embodiment, the gain mediumof the laser compensate at least partly for a small change in absorptiondue to deterioration. In this event, this may result in stable poweroutput relative to the deterioration; however, with an increase innoise, such as an increase in Relative Intensity Noise (RIN).

In one embodiment the fiber forms part of a laser with an operatingwavelength higher than 600 nm, such as higher than or equal to 800 nm,such higher than or equal to 1000 nm, such higher than or equal to 1064nm, such higher than or equal to 1150 nm, such higher than or equal to1300 nm, such higher than or equal to 1550 nm. For such a fiber anincrease in absorption in the visible may occur as a function ofoperating time, in the absence of loading according to the invention,similar to that occurring in microstructures fiber in supercontinuumgeneration. As argued above, such absorption may influence theperformance of the laser. By way of the invention such problems may beprevented, reduced and/or repaired so that a fiber according to theinvention is applied for part of or the entire optical path of thelaser. In one embodiment loading is performed in a manner similar tothat of the microstructured fiber for supercontinuum or the fiber fortransport.

In one embodiment the fiber forms part of an optical amplifier. Inoptical amplifiers high optical densities may arise, so that one willobserve that deterioration causing increased absorption occurs. This maybe prevented, reduced and/or repaired by way of the invention so that afiber according to the invention is applied for part of or the entireoptical path of the amplifier. For lasers as well as amplifiers it iswithin the scope of the invention that the fiber according to inventionis active and/or passive fiber.

FIG. 15 is a schematic cross-sectional view of an optical fiber 100according to an embodiment of the invention. The drawing is not made toscale, and is not intended to illustrate the relative diameters andthicknesses of the core material 110 and the cladding 112. The opticalfiber may be a microstructured fiber and include a pattern (not shown inFIG. 15) of holes or doped glass, extending in the longitudinaldirection (i.e., into the page) of the optical fiber 100.

1. A method of making a microstructured optical fiber having a corecomprising a core material comprising hydrogen and/or deuterium bound tothe core material and a cladding comprising a cladding material, whereat least a part of the core comprises silica or doped silica and thecore having a Germanium content of less than 0.001 at %, where themicrostructured optical fiber is a nonlinear fiber in that themicrostructured fiber can guide light for at least a range ofwavelengths λ_(min) to λ_(max) and for a mode field diameter (MFD) ofthe fundamental mode over at least a part of the range, the fraction(MFD)/λ is less than or equal to 5, the method comprising the steps ofloading the core material and the cladding material with hydrogen anddeuterium at loading temperature equal to or more than 120° C.;annealing the fiber at temperature T_(anneal) of more than 80° C. for aperiod of more than 1 hour; pumping the fiber for a period of more than12 hours with radiation having an average power of 1 W or more and apeak power density within the fiber equal to or higher than 100 W/μm²;reducing the temperature of the fiber and storing the fiber at thereduced temperature for a period of time before the step of pumping thefiber; and wherein the method allows the hydrogen and the deuterium tobecome bound to the core material and the cladding material.